Automatically skip a step if its attachTo.element isn't on the page

[jabbett]

Shepherd.js will insert raw, unstyled content into your page if a step's attachTo.element isn't present. Beyond the disappointing user experience, this also makes tours brittle because they have to be so tightly coupled with whatever particular dynamic elements are visible to a user.

There is an undocumented showOn option in Shepherd (usage example) that will let us check first whether the element is present. If not, we can skip to the next step.

I propose we make this behavior Abraham's default.